From 90b8f15d8c20b6484d93a837da9a2999aa70b848 Mon Sep 17 00:00:00 2001
From: adroitwhiz <adroitwhiz@protonmail.com>
Date: Tue, 4 Feb 2020 22:32:04 -0500
Subject: [PATCH 1/2] Un-premultiply extracted drawables

---
 src/RenderWebGL.js      | 3 ++-
 src/ShaderManager.js    | 5 +++++
 src/shaders/sprite.frag | 5 +++++
 3 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/src/RenderWebGL.js b/src/RenderWebGL.js
index 56c7c49..6f1c986 100644
--- a/src/RenderWebGL.js
+++ b/src/RenderWebGL.js
@@ -1124,7 +1124,8 @@ class RenderWebGL extends EventEmitter {
         gl.clear(gl.COLOR_BUFFER_BIT);
         try {
             gl.disable(gl.BLEND);
-            this._drawThese([drawableID], ShaderManager.DRAW_MODE.default, projection,
+            // ImageData objects store alpha un-premultiplied, so draw with the `straightAlpha` draw mode.
+            this._drawThese([drawableID], ShaderManager.DRAW_MODE.straightAlpha, projection,
                 {effectMask: ~ShaderManager.EFFECT_INFO.ghost.mask});
         } finally {
             gl.enable(gl.BLEND);
diff --git a/src/ShaderManager.js b/src/ShaderManager.js
index a71ef2d..4fb3da5 100644
--- a/src/ShaderManager.js
+++ b/src/ShaderManager.js
@@ -158,6 +158,11 @@ ShaderManager.DRAW_MODE = {
      */
     default: 'default',
 
+    /**
+     * Un-premultiply alpha. Useful for reading pixels from GL into an ImageData object.
+     */
+    straightAlpha: 'straightAlpha',
+
     /**
      * Draw a silhouette using a solid color.
      */
diff --git a/src/shaders/sprite.frag b/src/shaders/sprite.frag
index 13fe19d..b271fd2 100644
--- a/src/shaders/sprite.frag
+++ b/src/shaders/sprite.frag
@@ -210,6 +210,11 @@ void main()
 	#endif // DRAW_MODE_colorMask
 	#endif // DRAW_MODE_silhouette
 
+	#ifdef DRAW_MODE_straightAlpha
+	// Un-premultiply alpha.
+	gl_FragColor.rgb /= gl_FragColor.a + epsilon;
+	#endif
+
 	#else // DRAW_MODE_lineSample
 	gl_FragColor = u_lineColor * clamp(
 		// Scale the capScale a little to have an aliased region.

From a427461467290afb61eb6e4e46d9ffa3ecb75b24 Mon Sep 17 00:00:00 2001
From: adroitwhiz <adroitwhiz@protonmail.com>
Date: Thu, 26 Mar 2020 23:01:18 -0400
Subject: [PATCH 2/2] Clarify draw mode comments

---
 src/ShaderManager.js | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/ShaderManager.js b/src/ShaderManager.js
index 4fb3da5..3f0bc61 100644
--- a/src/ShaderManager.js
+++ b/src/ShaderManager.js
@@ -154,12 +154,12 @@ ShaderManager.EFFECTS = Object.keys(ShaderManager.EFFECT_INFO);
  */
 ShaderManager.DRAW_MODE = {
     /**
-     * Draw normally.
+     * Draw normally. Its output will use premultiplied alpha.
      */
     default: 'default',
 
     /**
-     * Un-premultiply alpha. Useful for reading pixels from GL into an ImageData object.
+     * Draw with non-premultiplied alpha. Useful for reading pixels from GL into an ImageData object.
      */
     straightAlpha: 'straightAlpha',